MOSCOW -- A Ukrainian air force pilot who was captured by separatist rebels last month has been arrested in Russia and charged with abetting the killing of two Russian journalists, Russian officials said on Wednesday.

Russia's Investigative Committee said in a statement Wednesday that 31-year-old pilot Nadezhda Savchenko is suspected of tipping off Ukrainian troops as to the whereabouts of the journalists as well as other unspecified "civilians" who were in a rebel-held area.

The investigators said Savchenko was a Ukrainian officer but she went on leave earlier this summer to join a voluntary battalion.

Savchenko is now in a Russian detention center but it is unclear how she got there. Moscow insists she crossed the border voluntarily, without documents and under the guise of a refugee. They said they have yet to find out why she went to Russia. Ukraine, however, recently raised the alarm about her disappearance, with the foreign ministry issuing a protest note.

Ukrainian Foreign Ministry spokesman Yevhen Perebyinis said Wednesday that "as far as we know she was held by terrorists" and called the claim that she had been freed and attempted to go to Russia "absurd."

Andriy Lysenko, a spokesman for Ukraine's National Security and Defense Council, was quoted by Ukrainian media as saying Wednesday that Savchenko had been kidnapped and the president has asked all authorities to work for her return.